Hello,
I've got a cheap aftermarket tachometer for my pickup truck.
There is two things which I don't like about it. At night, it lights up in this marine green color, which I can't stand. The second thing is the shift light, which is red. I would like to change it to a purple color, to match my pickups color.
It seems like the shift light would be easy to change, its just a matter of pulling it out of the glue, and soldering in another one.
However, I noticed the faceplate has a 'backing' to it, and there is wires going to it. I'm thinking the gauge face is lit by electroluminescence. Is there a way to make it light up white? I'm going to see if the gauge face is a tad bit transparent, I may try to put a led or two behind it, instead of the electroluminescence stuff.

Is there any voltages I have to figure out. I'm hoping you guys can help me. I don't want to fry the tachometer.

As balisong mentioned, it does sound like the green color is from an electroluminescent panel. That's the black thing behind the white dial. Of course, you risk ruining the tach by trying to remove the panel; you'd have to remove the tach needle, the screws, selector switch or pot, and perhaps the red LED.

The best color for instruments used at night is red. Red doesn't affect your night vision like other colors do, which is the reason red is used for cockpit instrument illumination in aircraft. If you use white, your night vision will be greatly reduced.

If you manage to get the EL panel out without destroying the movement, you may be able to use some wide-angle LEDs to light the dial from behind the tach, anchoring them near the circuit board. The further away from the rear of the dial the LEDs are, the more diffused and even the light will be.